The present invention relates to a method of assessing operation of an internal combustion engine common-rail injection system.
As is known, of the various problems that can occur in a common-rail injection system, the worst and most dangerous are leakage of the high-pressure circuit, which results in fuel leakage in the form of a very fine spray, and one or more of the injectors jamming in the open position.
On the one hand, high-pressure fuel leakage may cause a fire if the fuel spray should strike particularly hot engine surfaces; and, on the other, a jammed-open injector results in continuous fuel supply to the cylinders, in turn resulting, not only in excessive fuel consumption, but also in abnormal combustion characterized by pressure peaks and a considerable temperature increase in the cylinders.
Such defects can only be tolerated so long without causing serious damage to the engine, e.g. to the connecting rod, piston or injector nozzles, and may immediately impair operation and the safety of the vehicle.
To prevent this from happening, diagnostic units were proposed for detecting hazardous situations and which act on the injection system to immediately cut off fuel supply to the injectors and so immediately stop the engine.
In common-rail injection systems, however, the low-pressure circuit is also subject to fuel leakage caused, for example, by fine cracks in the low-pressure conduits or by faulty low-pressure circuit parts. Such leakage, however, is not as serious as that caused by fuel spray or a jammed-open injector, by not immediately impairing operation and the safety of the vehicle, which, in these cases, in fact, can safely be driven at least to the nearest repair shop.
Known diagnostic units, however, were unable to discriminate between high-pressure circuit fuel leakage caused, for example, by a jammed-open injector, and low-pressure circuit leakage caused by a generic fault in the low-pressure circuit. As a result, even in the case of minor nonhazardous faults in the low-pressure circuit, known diagnostic units immediately disabled the vehicle, thus causing considerable inconvenience to the driver, out of all proportion to the immediate danger involved.
Diagnostic units have therefore recently been proposed, designed to discriminate between injection system fuel leakage caused by a jammed-open injector, and leakage caused by a generic fault in the injection system.
The Applicant""s European Patent Application EP-0785349, for example, describes a diagnostic unit for determining a jammed-open injector condition using, among other things, an accelerometer signal related to the intensity of vibration on the engine and generated by an accelerometer sensor on the engine block. More specifically, the diagnostic unit compares the amplitude of the accelerometer signal with a first reference value; compares with a second reference value the engine angle value at which the amplitude of the accelerometer signal exceeds the first reference value; and determines a jammed-open injector condition according to the outcome of the two comparisons.
The Applicant""s European Patent Application EP-0786593, on the other hand, describes a fuel catch structure for determining leakage from the injector fuel supply conduits. More specifically, the structure comprises a number of sleeves made of elastomeric material, surrounding the injector supply conduits, and for catching any fuel leaking from the conduits; a catch header connected to the sleeves and for receiving any fuel leaking from the conduits and conveyed by the sleeves; a fluid sensor located beneath the catch header and for generating a leak signal indicating the presence of fuel in the catch header; and an alarm circuit connected to the fluid sensor and for generating an alarm signal when the catch header contains fuel.
While affording numerous advantages, particularly as regards efficient detection of the above fuel leakage conditions, both the solutions described have one drawback preventing their advantages from being fully exploited.
That is, both conditionsxe2x80x94fuel leakage caused by a jammed-open injector and fuel leakage from the supply conduitsxe2x80x94are determined using additional dedicated elements nor normally provided on the vehicle, such as an accelerometer sensor and the catch structure described above, which, besides costing money to manufacture or purchase and assemble, also call for periodic maintenance.
It is therefore an object of the present invention to provide a method of assessing operation of a common-rail injection system, and which provides, in a straightforward, low-cost manner, for discriminating between high-pressure circuit fuel leakage and leakage caused by a generic fault in the low-pressure circuit, with no need for additional elements other than those already provided on the vehicle.
According to the present invention, there is provided a method of assessing operation of a common-rail injection system of an internal combustion engine; said injection system comprising a number of injectors, a high-pressure circuit supplying high-pressure fuel to said injectors, and a low-pressure circuit supplying fuel to said high-pressure circuit; characterized by comprising the steps of:
hydraulically isolating said high-pressure circuit from said low-pressure circuit and said engine; and
assessing operation of said injection system as a function of the fuel pressure drop in said high-pressure circuit.